Abstract:

This article introduces the structure and properties of carbon fibers, as well as common methods for their surface treatment. It also summarizes the application fields of carbon fibers and the current status and trends of carbon fiber development in China.

1. Introduction

Carbon fiber is a fibrous carbon material with a density lower than that of metallic aluminum but with strength exceeding that of steel. It also features corrosion resistance and high modulus. Possessing both the inherent "hard" characteristics of carbon materials and the processability of textile fibers (being "flexible"), it represents a new generation of dual-use (military and civilian) advanced material, widely used in aviation, aerospace, transportation, sports and leisure goods, medical devices, machinery, textiles, and other fields. The carbon fiber industry plays an important role in the upgrading of pillar industries in developed countries and even in improving the overall quality of national economies. It is also of great significance for the industrial restructuring and upgrading of traditional materials in China [1].

1.1 Structure of Carbon Fibers

Carbon fibers have the basic structure of graphite, but not an ideal graphite lattice structure; rather, they possess what is known as a turbostratic graphite structure (see Figure 1-1). The basic units forming the polycrystalline structure are hexagonal carbon atom layer lattices, which form layer planes. Within the layer planes, carbon atoms are bonded by strong covalent bonds with a bond length of 0.1421 nm; between the layer planes, weak van der Waals forces act, with interlayer spacing ranging from 0.3360 nm to 0.3440 nm. The carbon atoms between layers have no regular fixed positions, resulting in uneven layer edges. Compared to the graphite structure, the carbon atom layers in carbon fibers undergo irregular translation and rotation, but the hexagonal covalent-bonded carbon atom layers are essentially aligned parallel to the fiber axis, giving the fiber an extremely high axial tensile modulus. In the turbostratic graphite structure, graphite layers are the most fundamental structural units, intersecting with each other. Several to dozens of layers form graphite crystallites, which in turn form fibrils approximately 50 nm in diameter and several hundred nanometers in length. Finally, these fibrils form individual carbon fiber monofilaments, typically 6–8 μm in diameter.

1.2 Formation of Carbon Fibers

During the formation of carbon fibers, various微小 defects form on the surface. This is because during the carbonization of precursor fibers, a large number of elements and various gases (such as CO₂, CO, H₂O, NH₃, H₂, N₂) are generated and escape, leading to voids and defects on the fiber surface and interior. Particularly when gas evolution is too violent at a certain stage, the voids and defects formed on the fiber surface and interior become more severe. The main defects observed in carbon fibers include five types: central holes, biconical voids, inclusions, needle-like pores, and surface cracks. The microcrystalline basal planes around surface defects conform to the shape of the defect, and the region of disordered orientation around the defect increases. In carbon fibers, the carbon atoms at the edges of graphite layers and those at defective sites on the surface differ from the intact basal carbon atoms within the layers. Basal carbon atoms within the layers experience symmetrical forces, have high bond energy, and exhibit low reactivity; carbon atoms at surface edges and surface defects experience asymmetrical forces, possess unpaired electrons, and are more active. Therefore, the surface activity of carbon fibers is related to the number of carbon atoms at edges and defect sites.

1.3 Properties of Carbon Fibers

Carbon fibers have low density, light weight, good electrical conductivity, are non-magnetic, possess electromagnetic wave shielding capabilities, and exhibit good X-ray transmittance. In recent years, due to decreasing carbon fiber costs and advances in composite material manufacturing technology, they have become a research hotspot for electromagnetic shielding composites. The bulk chemical composition of carbon fibers includes elements such as C, N, O, H, and trace metal impurities, while the surface chemical composition is C, O, H. Additionally, some polar reactive groups such as ketone, carboxyl, and hydroxyl groups exist on the surface, but their quantity is very small. As a result, untreated carbon fibers have a smooth surface, low reactivity, small specific surface area (generally less than 1 m²·g⁻¹), large wetting angle in water, hydrophobicity, and poor bonding and dispersion properties. Taking advantage of the fact that carbon fibers can be oxidized by oxidizing agents and by oxygen in air at high temperatures, the surface carbon elements can be oxidized into oxygen-containing groups, thereby improving the interfacial adhesion, wettability, and chemical stability of carbon fibers.

2. Research Progress in Surface Treatment of Carbon Fibers

During preparation, carbon fibers undergo carbonization treatment in a high-temperature inert gas atmosphere. As non-carbon elements escape and carbon becomes enriched, the number of active functional groups on the carbon fiber surface decreases, and the wettability with the matrix resin deteriorates. Furthermore, to improve the tensile strength of carbon fibers, surface defects must be minimized as much as possible, resulting in a small specific surface area. This smooth surface leads to a poor anchoring effect with the matrix, reducing the interfacial strength of carbon fiber composites and limiting the full play of the high performance of carbon fibers. Therefore, to improve the interfacial adhesion between carbon fibers and matrix materials and fully utilize the high strength and high modulus characteristics of carbon fibers, surface modification of carbon fibers is necessary to enhance wettability and adhesion with the matrix, thereby improving the interfacial bonding performance of the composite.

Surface modification of carbon fibers can achieve the following three effects:

• Prevent the formation of weak interface layers. Weak interface layers mainly include adsorbed impurities, release agents; oxide layers, hydrate layers formed during interface aging; and air layers trapped due to insufficient wetting with the matrix.

• Generate a surface morphology suitable for adhesion, creating凹凸 on the reinforcement material surface to improve interfacial bonding performance through the anchoring effect.

• Improve the affinity between the resin and the reinforcement material by coating the reinforcement material surface with a moderately polar covering agent, or by chemical treatment on the surface to introduce functional groups, thereby improving interfacial bonding performance.

Currently, the main methods used for surface modification of carbon fibers include oxidation treatment, coating treatment, plasma treatment, chemical vapor deposition treatment, surface grafting treatment, and supercritical fluid treatment.

2.1 Gas-phase Oxidation Treatment

Oxidation treatment is an important approach to improve and regulate the surface characteristics of carbon fibers. Through oxidation treatment, oxygen-containing groups such as carboxyl, hydroxyl, and carbonyl groups can be generated on the fiber surface, allowing chemical reactions between the fiber and resin matrix to form interfacial bonding. However, this method can also damage the structure of carbon fibers and affect their physical and chemical properties, so the oxidation time must be carefully controlled. Oxidation treatment mainly includes three methods: gas-phase oxidation, liquid-phase oxidation, and electrochemical oxidation. Gas-phase oxidation uses oxidizing gases to oxidize the fiber surface, introducing polar groups (such as -OH, etc.) and providing appropriate roughness to improve the interlaminar shear strength of composites. When using air oxidation, the oxidation temperature has a significant effect on the treatment outcome. J. Li et al. [2-3] treated carbon fibers using air oxidation and ozone oxidation, respectively, and then polymerized them to produce carbon fiber/polyetheretherketone (PEEK) composites. The results showed that after ozone oxidation, the -COOH content on the carbon fiber surface increased significantly. With an oxidation time of 3 minutes, the interfacial shear strength (IFSS) of the CF/PEEK composite increased by 60% compared to the untreated one. Compared with air oxidation treatment, ozone oxidation treatment was more effective.

Electrochemical oxidation generally involves using carbon fibers as the anode in an electrolyte solution, controlling the surface oxidation condition by changing parameters such as reaction temperature, electrolyte concentration, treatment time, and current density. Like other oxidation treatments, electrochemical oxidation introduces various functional groups (ester, carboxyl, hydroxyl, etc.) onto the fiber surface, thereby improving fiber wetting, adhesion characteristics, and bonding with the matrix, significantly increasing the mechanical properties of carbon fiber-reinforced composites. Currently, there are many reports on the electrochemical oxidation of carbon fiber surfaces. The content mainly involves the influence of oxidation conditions, the properties and morphology of the carbon fiber surface after oxidation, and oxidation mechanisms. Jie Liu et al. [4] electrochemically oxidized carbon fibers in a mixed (NH₄HCO₃)/(NH₄)₂C₂O₄·H₂O electrolyte. The results showed that oxygen- and nitrogen-containing functional groups on the carbon fiber surface increased significantly; not only did the tensile strength of carbon fibers increase by 17.1%, but the interlaminar shear strength (ILSS) of the carbon fiber composite also increased by 14.5%. Soo-Jin Park et al. used a composite amine electrolyte to perform surface amination treatment on PAN-based carbon fibers, achieving IFSS and ILSS values of 117 GPa, 87 GPa and 107 GPa, 103 GPa, respectively.

2.2 Plasma Treatment

Plasma is an aggregate state of matter containing a sufficient number of positively and negatively charged particles with approximately equal charges. Using plasma oxidation to modify fiber surfaces typically refers to the physical and chemical action of non-polymerizing gases on the material surface. Non-polymerizing gases can be either reactive or inert gases. Plasma oxygen is commonly used, which has high energy and strong oxidizing power. When it impacts the carbon fiber surface, it can oxidize defects such as crystal corners and edges or double-bond structures into oxygen-containing active groups. Huang Yudong et al. treated carbon fibers with plasma air and then produced carbon fiber/phenolic composites. When the treatment time was 20 minutes, the ILSS and the interfacial micro-debonding force between the single fiber and the matrix resin increased by 52.8% and 56.5%, respectively, and the interfacial bonding performance of the final product increased by more than 40%. Xiong Jie et al. treated carbon fibers with cold plasma oxygen, and the maximum fracture load and toughness index of their CFRP-cement mortar increased significantly. Kingsley Kin Chee Ho et al. [5] adopted a new treatment method involving intermittent or continuous single-sided or double-sided fluorination of carbon fibers using plasma, introducing fluorine groups onto the carbon fiber surface.

2.3 Coating Treatment

Coating treatment involves applying a certain polymer onto the fiber surface to change the structure and properties of the composite interface layer. Surface coating serves the following functions: the coating can protect fibers from damage, improve fiber bundling, and help utilize fiber strength; the coating can change the surface properties of the fiber and improve fiber wettability with the resin matrix; reactive functional groups in the coating facilitate chemical bonding between the fiber surface and the resin matrix; the coating can prevent the loss of surface activity after surface treatment. Tamaki Melanoma et al. [6] coated a polyimide (PI) nanocoating approximately 100 nm thick onto the surface of T1000 carbon fibers. When the carbon fiber bundle was stretched, the PI nanocoating helped prevent the propagation of surface defects on the carbon fibers and reduce stress concentration, effectively enhancing the tensile strength of the carbon fibers.

3. Applications of Carbon Fibers

3.1 Aerospace Field

Carbon fiber composites have a series of advantages, including high specific strength and specific modulus, good fatigue resistance, and excellent dimensional stability. They serve as a fundamental material for the development of new-generation weaponry and are widely used as structural materials for aircraft and spacecraft. Examples include primary structural materials for aircraft main wings, tail wings, and fuselages; secondary structural materials such as ailerons, rudders, elevators, interior materials, floor materials, beams, and brake pads; helicopter blades; rocket exhaust cones, engine covers, etc.; satellite structural bodies, solar panels and antennas, launch vehicles, and missile casings.

3.2 Building Reinforcement Field

The specific strength of fiber-reinforced composites is much higher than that of steel, and their specific modulus is generally higher than that of steel as well. This excellent mechanical performance has led to their widespread application as reinforcement and repair materials for civil engineering structures in Japan, the United States, Europe, and other countries and regions. Carbon fiber materials have an elastic modulus comparable to that of steel while exhibiting a tensile strength ten times higher than ordinary steel. Their corrosion resistance and durability are also excellent. Therefore, when using carbon fibers to reinforce concrete structures, no additional bolts or rivets are required for fixing. The corrosion resistance and durability are outstanding, the disturbance to the original concrete structure is minimal, and the construction process is simple and convenient.

Conclusion

In summary, the various surface treatment methods for carbon fibers each have their own characteristics. Among non-oxidation methods, vapor deposition and plasma methods are still in the laboratory stage both domestically and internationally and have not yet achieved industrial production; coupling agent coating and polymer coating methods show insignificant effects. Among oxidation methods, liquid-phase oxidation is only suitable for batch operation; the reaction time for gas-phase oxidation depends on the type of carbon fiber and the desired degree of oxidation; gas-liquid dual oxidation is difficult to control. Relatively speaking, electrochemical oxidation has the most advantages. It not only greatly improves the surface wettability and reactivity of carbon fibers but also features mild treatment conditions that are easy to control. The fiber surface treatment is uniform, and the method is easily integrated with carbon fiber production lines, offering broad prospects for application in industrial carbon fiber production.

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